Device having a radial partition in a double walled pipe intended for great depths

ABSTRACT

A device for arresting the propagation of a buckle appearing on an outer tube of a double-walled rigid pipe intended to transport hydrocarbons and consisting of two coaxial tubes. The double-walled pipe is of the reelable type and intended for very great depths and consists of a thick and rigid annular transverse partition fastened to the inner tube and to the outer tube. The partition is joined to the outer tube and to the inner tube by way of an outer and inner sleeve having the same diameter as the outer and inner tubes and fastened to the latter.

CROSS REFERENCE TO RELATED APPLICATION

This is a divisional of U.S. patent application Ser. No. 09/768,634,filed Jan. 24, 2001, now U.S. Pat. No. 6,513,551 in the name of Bill G.LOUIS and Mike BELL and entitled DEVICE HAVING A RADIAL PARTITION,ESPECIALLY FOR ARRESTING THE PROPAGATION OF A RADIAL BUCKLE IN ADOUBLE-WALLED PIPE INTENDED FOR GREAT DEPTHS.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device having a radial partition,especially for arresting the propagation of a radial buckle in adouble-walled pipe consisting of two, respectively inner and outer,coaxial metal tubes separated by an annular space, this pipe being arigid pipe for transporting fluids such as hydrocarbons, and intendedfor very greater depths.

2. Relevant Art

A rigid pipe or tube is laid on the seabed usually from a so-calledpipelaying gauge. The laying is called S-laying when the pipe adopts theshape of an S between the pipelaying gauge and the seabed and it iscalled J-laying when the pipe adopts the shape of a J. In the lattercase, a guide ramp is provided on the pipelaying gauge, which ramp maysometimes be partially immersed in the water.

The rigid pipe to be laid is stored on the pipelaying gauge either inpipe sections of a given but relatively short length, the pipe sectionsbeing joined together as the laying progresses, or it is wound as agreat length on a reel, the pipe then being unreeled from the reelduring the laying operation. These laying operations are described inthe API (American Petroleum Institute) document “Recommended Practice 17A” from 1987.

When the pipe has left the gauge and while the pipe is being laid, it isimportant that the latter undergoes no plastic deformation in bending,which would result in ovalization of the pipe causing a “weeksingularity” which would be conductive to the initiation of a collapse.Moreover, when the pipe is laid on the seabed at great water depths(typically greater than 300 m and possibly up to 2000 m and more), thehydrostatic pressure exerted on the pipe may be sufficient to initiate aradial buckle which has a tendency to propagate as a wave along thepipe, in both directions. Of course, the buckle will form preferentiallyat a “weak singularity” when one exists on the pipe. When the buckleoccurs, it is then necessary to replace at least that section or portionof the pipe comprising the buckled or collapsed region. The bucklepropagation pressure is given by the formula:

26×σ₀×(T/D)^(2.5)

where σ₀ is the yield stress of the steel, T is the thickness of thepipe and D is the external diameter of the pipe. To resist thepropagation of a buckle, the corresponding pressure must be greater thanthe hydrostatic pressure.

To prevent the propagation of a local buckle or local buckles, it hasbeen proposed to provide the pipe with certain devices or means, calledbuckle arrestors. The API Recommended Practice 1111 gives variousrecommendations and formulae which indicate from which depth thearrestors are recommended, necessary or strictly indispensable.

Such devices are firstly proposed within the context of single-walledrigid pipes.

According to a first solution, a cylinder is placed inside the pipe.Thus, it is proposed in U.S. Pat. No. 3,747,356 to link a cylinder to acable, to lodge the cylinder inside a pipe section and then tosimultaneously unreel the pipe and the cable so as to keep the cylinderin the pipe section while the latter is being laid, until the pipe comesinto contact with the seabed. The cylinder is then brought back up so asto be lodged in another pipe section to be laid, which is joined to theprevious section. Consequently, any buckle likely to occur, when layingthe pipe, between the pipelaying gauge and the seabed is immediatelyarrested and therefore not allowed to propagate along the pipe sections.However, such an arrangement provides no solution or effectiveness forarresting buckles likely to be propagated after the pipe has beenfinally laid on the seabed.

According to a second solution, an inner, or preferably outer,reinforcing collar (possibly in two parts, constituting a “clamp”) isused. Thus, in U.S. Pat. No. 3,768,269, it is proposed to locallyincrease the stiffness of the pipe by placing, at regular intervals, forexample at intervals ranging between 100 m and 500 m, reinforcingcollars whose length ranges between 1 m and 2.5 m. Such a solution isvalid only for pipes laid in sections since the reinforcing collars canbe mounted and fastened in the factory to the pipe sections and thentransported by the pipelaying gauge to the laying site. When the pipe islong and would onto a storage reel, it then becomes virtually impossibleto wind the pipe with its reinforcing collars onto a reel since theywould result in straight or almost straight portions that cannot bedeformed when winding the pipe onto the storage reel. In order tomitigate this difficulty, it is conceivable to mount and fasten thereinforcing collars during the laying operations. However, it would thenbe necessary to interrupt the laying, at regular intervals, so as tomount and fasten the reinforcing collars. According to an alternativesolution known through this same patent or through documents GB1,383,527 or U.S. Pat. No. 5,458,441, the localized reinforcement maytake the form of a thicker intermediate sleeve welded to the ends of thepipe.

According to a third solution, a spiralled rod is used on the externalwall of the pipe. Thus, to allow the pipe to be wound onto a reel, U.S.Pat. No. 4,364,692 proposes to wind a rod tightly around the pipe so asto form a certain number of turns which can be welded at their ends tothe rod itself and/or to the pipe.

According to another embodiment, the turns may be individual turns, bywelding their two ends and regularly spacing them apart along thatportion of the pipe to be reinforced. As long as the pipe is asingle-walled pipe, the increase in the diameter in the reinforcedportions may be acceptable. However, when the pipe is of thedouble-walled or pipe-in-pipe type, that is to say comprising anexternal tube or liner pipe which is slipped over the internal tube, theincrease in the diameter of the liner tube is unacceptable whentransporting and storing long lengths of double-walled pipe.

In addition, when the rigid pipe to be laid is manufactured in longlengths on land and then wound onto a reel on the pipelaying gauge, thesolutions recommended in the aforementioned documents are notappropriate as they use either long reinforcing collars, having a lengthof about 1 to 2.5 m, as in U.S. Pat. No. 3,768,209, or the winding of areinforcing rod around the rigid pipe, as in U.S. Pat. No. 4,364,692.

For the purpose of solving these problems and of obtaining double-walledpipes that can be wound, despite the propagation arrestors, theApplicant has already proposed particular devices.

According to Application FR 99/08540, a portion of flexible pipe iswelded to the internal wall of the external tube in order to form aflexible propagation arrestor.

According to Application FR 99/15216, the propagation arrestor consistsof an annular compartment filled with resin which is injected before orduring the laying and which can be cured only after the laying if thelength of the compartment for the resin is too great to allow winding inthe cured state.

In the field of double-walled metal pipes, devices of various kinds areknown which are placed between the inner and outer coaxial tubes andwhich are not intended to arrest the propagation of a buckle, but ratherto form compartments, to serve as a spacer or to join several sectionsof pipe together. For example, end-blocking systems, or bulkheads, fordouble-walled rigid pipes exist and are described especially in WO96/36831 and WO 98/17940. Such bulkheads cannot be likened topropagation arrestors since the elastic material from which they areproduced is not capable of transferring the stresses applied to theexternal tube on the internal tube.

Also known in a very different field, that of double-walledthermoplastic pipe assemblies, for example from documents U.S. Pat. Nos.5,141,260 and 4,786,088, are radial-partition spacers between the twowalls which make it possible to prevent the problems associated with thedifferential expansion of the two walls, which may cause axialdistortions that these spacers can contain. These pipe assemblies arenot intended to be submerged nor to be exposed to a surrounding pressureof the kind to induce radial buckling phenomena. Nor are these pipeassemblies intended to be wound.

It is apparent from this prior art that it is known, especially thanksto the Applicant, to have propagation arrestor devices on double-walledmetal pipes which remain reelable, but at the cost of degree ofstructural complication and therefore an increase in the cost.

SUMMARY OF THE INVENTION

The objective of the invention is to provide a propagation arrestordevice for a reelable double-walled pipe, which is simple to manufactureand to fit.

The objective of the invention is achieve within the framework of areelable double-walled metal pipe because the arrestor device consistsof a thick and rigid annular transverse partition fastened to the innertube and to the outer tube.

A solution is all the more surprising as it is particularly simple buthas never been considered, despite the constant need in the oilindustry, this need being all the more pressing since operations indeeper and deeper water make the propagation arrestors absolutelyindispensable.

A thick and rigid annular transverse partition has already been knownper se for a very long time, for example from the already-mentioneddocument U.S. Pat. No. 3,768,269, but this involved only a collarexternal to the single pipe and which therefore did not prevent the pipefrom being reeled.

The thickness of the partition according to the invention is typicallyless than 0.5 times the outside diameter of the pipe, unlike thearrestors for a double-walled pipe that have been known hitherto. Thethickness is even preferably less than 0.2 times the outside diameter.

In one embodiment, the partition is joined to the outer tube and/or tothe inner tube by means of a sleeve having approximately the samediameter as the said outer and/or inner tube and fastened to the latter.Advantageously, the device has a half cross section in the form of anelongate H. To avoid certain difficulties when inspecting the in situwelds, it may be made of steel, for example as a single casting, and maybe welded to the ends of the tubes of the pipe.

The fastening of the partition and/or the sleeve to the tubes may bedone by welding, adhesive bonding or even screwing.

When winding onto the reel, the inner tube will be subjected to a veryhigh local stress at the partition. This phenomenon, which mightpossibly result in the creation of cracks in the inner tube, may becombated by gradually and locally increasing the thickness of the innertube in the region provided for the partition.

The pipe according to the invention can be used at great depths, withoutother forms of propagation arrestors than those above, and without anymodification of the pipe (such as increasing the thickness of the outertube).

Another aspect of the invention provides a device with a radialpartition for a rigid double-walled pipe or tube for transportinghydrocarbons, the said pipe consisting of two, respectively inner andouter coaxial metal tubes separated by an annular space, this pipe beingintended for very great depths, characterized in that it consists of athick and rigid annular transverse partition joining an outer metalsleeve to an inner metal sleeve of the same diameter as the said outertube and the said inner tube respectively, these sleeves being designedto be welded to the inner tube and to the outer tube respectively,connection between the inner sleeve and the outer sleeve being providedby screwing.

Advantageously, the inner sleeve is offset axially with respect to theouter sleeve (that is to say the inner sleeve extends beyond the outersleeve on one side and is set back on the other), so as to facilitatethe laying by welding, by limiting the number of welds. Thispartitioning device is particularly advantageous for rigid pipes whichare not wound and are assembled offshore, since it reduces themanufacturing time. It gets round the problem of intermediate pieceswhich are needed for a partition device in the form of a single piece orin the form of two pieces which are welded together, and it also has theadvantage of allowing the position of the external tube to be easilyadjusted with respect to the internal tube, obviating manufacturingclearances.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and features will become more apparent on reading thefollowing description of the invention, with reference to the appendeddrawings in which:

FIG. 1 is a longitudinal cross-sectional view of a portion of adouble-walled rigid pipe fitted with a propagation arrestor deviceaccording to the invention;

FIGS. 2, 3 and 4 show schematically, in cross section, three steps offitting the device of the invention during the manufacture of the pipe;

FIGS. 5 and 6 show, in longitudinal cross section, two other embodimentsof the propagation arrestor of the invention;

FIG. 7 shows, in two longitudinal half-cross sections, two variants ofthe device in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The double-walled rigid pipe 1 of longitudinal axis A, shown partiallyin FIG. 1, comprises an inner wall or tube 2 (the “flow pipe”), thediameter and the nature of the material of which are chosen according tothe fluid flowing in the said inner tube, especially according to thetemperature and pressure of the said fluid, and an outer wall or tube 3(the “carrier pipe”) which is slipped over the inner tube 2. The outertube 3 generally has an outside diameter D which is larger in dimensionsthan the inner tube 2 in order to allow a thermal insulation to beplaced in the annular space 5 and has a thickness which makes itpossible to withstand the hydrostatic pressure which is exerted on thesaid outer tube 3. The rigid pipe 1 generally includes spacers (notshown) which are fastened to the external wall 4 of the inner tube 2 andwhich are lodged in the annular space 5 provided between the outer tube3 and the inner tube 2.

The propagation arrestor device 10 essentially comprises an annularpartition 11 fastened to the tubes 2 and 3. The thickness E of thepartition is quite small compared with the diameter D, for example about1 to 2 tenths of D. In one embodiment, E=30 mm and D=270 mm.

The partition 11 is a piece with a short outer sleeve, approximatelyhaving the same thickness and the same diameter as the outer tube 3. Thepartition 11 is screwed onto an external thread 13 of an inner sleeve 14having approximately the same thickness and the same diameter as theinner tube 2. However, the thread 13 is provided in the middle of aregion 15 of the sleeve, the thickness of which gradually increasestowards the outside and towards the middle. The sleeves 12 and 14 arefastened to the inner 2 and outer 3 tubes by welds 16 and 17. The thread13 is advantageously an ACME-type thread (a trapezoidal thread) widelyused in the oil sector for joining sections of pipe and allowing goodload transfer. The thread 13 may be placed on any part of the partition11, not necessarily at the point where it is joined to one or other ofthe sleeves.

Preferably, the inner sleeve 14 and the outer sleeve 12 are offsetaxially (for example by 100 mm) in order to facilitate the fitting whichwill now be explained in FIGS. 2 to 4. The double-walled pipe ismanufactured in a known manner by joining a section of a certain length(for example 12 m). When an arrestor has to be interposed, approximatelyevery 200 m of pipe, the process starts by welding, at 16, the threadedinner sleeve 14 to the inner tube 2 (FIG. 2); the assembly consisting ofthe partition 11 and the outer sleeve 12 is screwed thereon and theouter sleeve 12 is welded at 17 to the outer tube 3.

Since the outer sleeve 12 is offset with respect to the inner sleeve 14,it is not necessary to use an intermediate tube (two half-shells) inorder to joint the outer sleeve 12 to the outer tube 3. It will beclearly understood that, when the device consists of a single piece orof several welded pieces, an intermediate tube will be necessary, theexternal sleeve 12 being shorter than the internal sleeve 14 in order toallow the internal sleeve 14 to be welded to the inner tube 2. Inaddition, the screwed device allows optimum adjustment of the positionof the outer sleeve 12 with respect to the outer tube and thereforemakes it possible to take up any manufacturing clearances.

The partition may also be welded at the thread in order to ensureleaktightness, this weld not having to take the loads and thereforebeing simple to produce. Next, the inner tube 2′ of a new section ofpipe is pulled out, by releasing it from the outer tube, by an amountlong enough to allow it to be welded at 16′ to the inner sleeve 14 ofthe arrester fitted (FIG. 4), the outer tube 3′ (shown in dotted lines)of the new section is then pulled back over the inner tube 2′ justwelded, in order to weld it at 17′ to the outer sleeve 12, and theprocess may continue with the conventional manufacture of severalsections of pipe until the next propagation arrester is fitted.

The arrestor described above is made entirely of steel (possibly indifferent grades for the inner sleeve and for the outer sleeve) and mayconstitute a thermal bridge between the external environment of thedouble pipe and the inner tube. Right at each arrestor, this thermalbridge may create a cold spot which results in the formation of paraffinwax. To avoid this problem, a thermal insulation based on a ceramic maybe created between the insulated parts of the double pipes.

For this purpose, the partition 11, (FIGS. 5 and 6) may be made ofceramic and the sleeves 12 and 14 of steel. The partition may beadhesively bonded to the tubes with an anaerobic adhesive which ensuresleaktightness. The embodiment in FIG. 5 has retained a steel thread 18engaging with the thread 13 of the inner sleeve 14, whereas in theembodiment in FIG. 6 the arrestor 10 is made as one piece by directlybonding the partition 11 to the sleeves 12 and 14. The thermalconductivity of the ceramic must be less than 0.5 W/K.m and preferablyless than 0.2 W/Km.

FIG. 7 shows two alternative forms of the embodiment in FIG. 1. Theupper part of the figure shows the case in which the partition 11 iswelded directly to the sleeves, respectively the outer sleeve 12 andinner sleeve 14 (or even to the tubes themselves) and the lower part ofthe figure shows the case in which the entire device 10 is produced as acasting which includes the sleeves 12, 14 and the partition 11. However,these two embodiments do not allow the advantageous fitting as in FIGS.2 to 4 and require the use of an intermediate linking piece, generallyconsisting of two half-shells, which will entail additional welds,namely two longitudinal welds and one circumferential weld.

What is claimed is:
 1. An underwater system to transport hydrocarbons,comprising: a plurality of pipe sections, each pipe section beingcomprised of an inner tube and an outer tube surrounding and spacedoutwardly from the inner tube so that an annular space is formed betweenthe outer and inner tubes; and a plurality of coupling devices toconnect coaxial adjacent pipe sections, at least one of the couplingdevices being comprised of: a linear inner sleeve fastened between theinner tubes of the adjacent pipe sections, the inner sleeve having amid-portion; a linear outer sleeve fastened between the outer tubes ofthe adjacent pipe sections, the outer sleeve having a mid-portion; and arigid transverse annular partition extending completely across theannular space and connected to the mid-portions of the inner and outersleeves, with the partition and at least one of the sleeves beingthreadably connected.
 2. The piping system of claim 1, wherein the outerand inner sleeves are formed of metal and are respectively fastened tothe outer and inner tubes of the coaxial adjacent pipe sections bywelding.
 3. The piping system of claim 1, wherein the outer and innersleeves are respectively fastened to the outer and inner tubes of thecoaxial adjacent pipe sections by welding.
 4. The piping system of claim1, wherein the outer sleeves of the coupling devices have outerdiameters that are the same as outer diameters of the outer tubes of thecoaxial adjacent pipe sections.
 5. The piping system of claim 4, whereinthe partition has an axial dimension less than 0.5 times the outerdiameters of the outer tubes.
 6. The piping system of claim 5, whereinthe partition has an axial dimension less than 0.2 times the outerdiameters of the outer tubes.
 7. The piping system of claim 1, whereinthe inner sleeves of the coupling devices have outer diameters that arethe same as outer diameters of the inner tubes of the coaxial adjacentpipe sections.
 8. The piping system of claim 1, wherein the inner andouter sleeves of the coupling devices are axially offset from oneanother.
 9. The piping system of claim 1, wherein the partition isformed of a material that is mechanically strong and thermally poorlyconductive.
 10. The piping system of claim 1, wherein the partition isformed of a ceramic material.
 11. The piping system of claim 1, whereineach of the inner sleeves includes two ends and a wall, the wall havinga first thicknesses in an area of the mid-portion of the inner sleeveand a second thickness in an area of the ends of the inner sleeve, thefirst thickness being greater than the second thickness.
 12. The pipingsystem of claim 11, wherein the wall includes a third thickness thatincreases gradually from the second thickness to the first thickness.